I've found the most effective way is to never share your comic in multiple "share your comic!" "comics under x subs!" threads all at the same time. You need to spread it out, because different people are on the forum at different times or on different days, and spamming comes across desperate and pathetic, like you're unable to get readers, which will make people perceive your comic as "low demand" or "less good".
Prioritise threads where you post sample pages. I pretty much never miss the "post your latest page [month]" threads; those threads are some of the most useful; a page is worth so much more than a description (I also frequently have found new comics to read on those threads). Threads where you post things like work in progress to finished pages or pick out the funniest or coolest page or funny panels are also great. They're more effort, but that's the point. Minimal effort copy-paste spam posting the link and a blurb in a bunch of threads all at once, then being completely silent the rest of the time and posting in no other threads is highly ineffective, especially because most people on "share your work" threads are only there to promote themselves.
